The field of the invention generally relates to semiconductor devices, and more particularly relates to thin film resistors and a method of fabricating such resistors.
As is well known, integrated circuits and thin film devices frequently require resistors as part of the circuitry, and thin film resistors are commonly used. Thin film resistors generally consist of a thin film of resistive material deposited such as by sputter deposition on a layer or substrate of insulative material with end contacts on the resistive material. The end contacts or interconnections are then connected to circuit components in a conventional manner.
There are a number of criteria by which the quality of thin film resistors are evaluated. For example, it is generally desirable that a thin film resistor have a minimum thickness such as 30 angstroms. When a thin film resistor is too thin, it may be unable to handle relatively large current densities during operation. It is further desirable that a thin film resistor have uniform thickness and properties to insure consistency and stability. Also, it is generally desirable that thin film resistors have a target or intended sheet resistance which is expressed in ohms per square. Further, it is normally desirable that thin film resistors have a very low temperature coefficient of resistance, or at least a temperature coefficient of resistance that is suitably matched to a particular application. For example, it may be desirable to have a temperature coefficient of resistance that is either positive or negative. The temperature coefficient of resistance defines how the sheet resistance varies with temperature. Therefore, a thin film resistor with a zero coefficient of resistance does not vary in resistance as the temperature changes.
It may be desirable to use certain resistive materials such as, for example, 85% chromium diboride with 15% silicon chromide by atomic weight. This particular resistive material and others can readily be used to fabricate thin film resistors having relatively low sheet resistances such as 1000 ohms per square and less. However, this material and others are not generally suitable if it is intended that the sheet resistance be relatively high such as, for example, 1500 ohms per square or higher. In particular, due to the inherent properties of this particular material and others, the thickness of a thin film resistor must be undesirably thin in order to attain relatively high sheet resistances. Not only are such film resistors unable to handle relatively high current densities during operation, but it is difficult to attain thickness uniformity with extremely thin films of material. A dopant could be used to increase the sheet resistance of such a material, but such action would also generally raise the temperature coefficient of resistance, and that would be undesirable as discussed above.